1. Field of the Invention
This invention relates generally to a rotary head device for mounting on a tape guide drum, and more particularly is directed to a rotary head device of the type that is capable of displacing a magnetic head or heads transversely in respect to recording tracks being scanned thereby in a magnetic recording and reproducing apparatus, such as, a video tape recorder, which is hereinafter referred to as a VTR.
2. Description Of The Prior Art
In existing VTRs of the helical scan type, a magnetic recording tape is wrapped helically about the peripheral surface of a cylindrical tape guide drum, and magnetic heads are rotated about the axis of the drum so as to scan recording tracks extending obliquely across the tape. During normal or standard recording operations, the magnetic recording tape is longitudinally advanced at a predetermined or standard tape running speed so that the rotary heads scan and record video signals in successive oblique recording tracks on the tape during successive revolutions of the heads. During normal reproducing or playback operations, the standard tape running speed is again employed and a servo control ensures that each rotary head precisely traces, and reproduces the video signal recorded in one of the recording tracks during each scanning of the tape by such head. However, during slow-motion or fast-motion playback operations, the tape running speed is either reduced or increased in respect to the standard tape running speed with the result that the scanning path or trace of each rotary head across the tape is angularly displaced from the recording track to be scanned. Further, irregularities in the rotation of the magnetic head or heads produce so-called jitters which generate a noise band on the display screen.
In order to compensate for deviations of the trace or scanning path of a rotary magnetic head from the recording track being scanned during slow-motion or fast-motion playback, rotary head devices have been provided to automatically displace a magnetic head transversely in respect to the recording track being scanned so as to ensure the precisely correct tracing of the recording track from one end to the other. Such known rotary head devices are generally of two types, namely, the so-called bimorph leaf type, and the moving-coil or electromagnetic type, respectively.
In the known rotary head devices of the bimorph leaf type, a bimorph piezoelectric or magnetically operated bending or pivoting leaf member is anchored, at one end, in respect to the tape guide drum and supports a magnetic head assembly at its free end so that the magnetic head is displaced, desirably in the direction extending transversely in respect to the recording track, in response to the application of a control voltage to the bimorph plate or leaf member. Such control voltage is generated so as to maintain the magnetic head in accurate scanning relation to the recording track being traversed thereby during the various different playback modes, that is, during normal playback as well as during slow-motion or fast-motion playback.
The rotary head devices of the bimorph leaf type have certain functional disadvantages that limit the use thereof in so-called professional VTRs and in digital VTRs which require relatively large displacements of the movably supported magnetic head or heads. In this respect, it is to be noted that the displacement of a magnetic head supported by a bimorph leaf member is inherently limited. If the length of the bimorph leaf member is increased in order to increase the maximum displacements of the magnetic head mounted at the free end of the leaf member, the frequency characteristic of the head mounting becomes undesirable. Further, increasing the displacement of the magnetic head supported by the bimorph leaf member reduces the useful life of the rotary head device as cracks develop in the bimorph material. Rotary head devices of the bimorph leaf type are also subject to hysteresis or drift, and thereby are difficult to control, particularly if the movably mounted magnetic head is used for recording as well as for playback. When rotary head devices of the bimorph leaf type have been designed to provide the relatively large displacements required for use in professional VTRs, a pair of bimorph leaves disposed in substantially parallel, spaced apart relationship have been employed for supporting a magnetic head or heads, for example, as in U.S. Pat. No. 4,441,128, issued Apr. 3, 1984, and having a common assignee herewith. However, such rotary head devices employing a pair of bimorph leaves are expensive, in part, due to the cost of providing the long bimorph leaves required for obtaining the desired large head displacements. Finally, rotary head devices of the bimorph leaf type are disadvantageous in that they require the application of high control voltages thereto for effecting the desired displacements of the movably supported heads.
Rotary head devices of the moving coil type have been proposed, for example, as disclosed in U.S. Pat. No. 4,212,043, issued July 8, 1980, in which a pair of substantially parallel, spaced apart leaf members which are flexible, at least in part, are fixed, at one end, to the rotary tape guide drum and are connected, at their other ends, to form a flexibly hinged parallel linkage assembly on which a magnetic head assembly is mounted for the desired displacements thereof upon angular displacements of the respective flexibly hinged parallel-linkage assembly. In such rotary head devices of the moving coil type, an electrical voice coil is mounted on the flexibly hinged parallel-linkage assembly to act as the movable element of a linear motor which further includes a permanent stator magnet fixed to the rotating drum so that, when suitably generated reversible currents of varying magnitudes are suitably supplied to the voice coil, the latter is electro-magnetically displaced relative to the stator magnetic in directions parallel to the drum axis for causing corresponding displacements of the magnetic head assembly. The foregoing known rotary head devices of the moving coil type are disadvantageous in that a shield must be provided for ensuring that the magnetic fields of the stator magnet and of the moving voice coil will not influence the magnetic head or heads and thereby interfere with the recording and reproducing functions. Since a linear motor is employed, providing a relatively large displacement of the movably supported head assembly requires a correspondingly large shield structure. Further, in the described rotary head devices of the moving coil type, since the direction of movement of the moving voice coil is parallel to the axis of rotation of the tape guide drum, centrifugal forces due to rotation of the drum act laterally on the voice coil and would strongly resist deflection of the flexibly hinged parallel-linkage assembly if the leaf members of the latter extended radially in respect to the axis of rotation of the drum. The centrifugal forces acting laterally on the voice coil also can disturb the desired precise positioning of the voice coil relative to the stator magnet and the tubular pole pieces associated therewith. Furthermore, the voice coil, in being directly connected with the flexibly hinged parallel-linkage assembly increases the mass of the structure being displaced with the magnetic head assembly and thereby interferes with the attainment of a desirable frequency characteristic.